Impedance is the total opposition to the flow of alternating current (AC) in an electrical circuit.  

Impedance varies with frequency so nominal impedance measurements can refer to average or minimum values.

The opposition of inductance and capacitance to alternating current is called reactance and is measured in ohms and has a phase angle of plus or minus 90 degrees. 

When this value is added to the resistance of a circuit the resultant sum is called impedance

source impedance 
(lower is better) 

this is the impedance of an output and is a measure of how easily power will flow from that output 

load impedance 
(higher is better) 

this is the impedance of an input and is a measure of how much power the input will draw 

Impedance is a complex measurement of a device's resistance to the flow of electrical energy. For microphones the impedance value refers to its output resistance at 1 kHz.  Low impedance mics 
(low Z = approx. 200 ohms) are preferable to high impedance ones 
(high Z = approx. 50,000 ohms) because they reflect noise more effectively and long cables (up to 200 ft.) can be used without the problem of picking up hum or losing high frequency information. 
Impedance matching is an important issue.
If the impedances of the various pieces of interconnected equipment are not compatible then signal degradation will occur such as high frequency attenuation as well as overall signal attenuation. This does not mean that impedances of these pieces of equipment should be identical.  If a microphone has a low impedance of 200 ohms, a typical mixer mic input or preamplifier should have a low impedance value of between 5 to 10 times or higher that of the mic impedance giving a value of between 1000 and 2000 ohms or higher for best results. Most professional audio equipment is designed to operate at low impedance (lo Z) and will not perform well in combination with high impedance microphones unless they provide special inputs designed for these types of mics. 

added by martin

Alternating current resistance measurement.  The total opposition ot the flow of alternating current or an audio signal. Measured in ohms. Sum of resistance, capacitive reactance and inductive reactance.  Many different impedances are found in audio systems. Speakers are low impedance devices (usually 8 ohms) - voltages will have large amounts of current (power = voltage X current) - so this works well as speakers need to do a great deal of real work.  Microphones are higher impedance as the power levels are very low.  Matching impedances does not mean that the output device has the same impedance as the input of the device it is connected but rather that their impedance is set for optimum signal strength and fidelity.  General rule of thumb is that with modern equipment the output impedance should be low as possible and the input impedance be as high as workable.  Impedance mismatch can result in improper frequency response, distortion, and circuit failure.  Important to note that professional microphones have roughly a 250 - 600 ohm impedance while cheaper microphones commonly found with cassette decks, etc. are high impedance (10,000 ohms) and are generlly not acceptable for recording purposes. Also important to note that electric guitars have a unique signal which is relatively high impedance ( 500KOhm - 1 MEG Ohm) and usually requires a guitar preamp (such as found in a guitar amp,etc.) or a “direct box” which will match the impedance to a standard mic input on a mixer for the best signal transfer.